The present invention relates to an improved method and apparatus for pumping liquids or various suspensions. The method, apparatus and rotor used in connection therewith are especially preferably suitable for pumping fiber suspensions of the paper and pulp industry at medium consistency (8-20%) and high consistency (over 20%). According to a preferred embodiment of the invention the method, apparatus and rotor used in connection therewith are suitable for pumping viscous and/or air-containing mediums. The method invention mainly relates to intensifying the pumping of liquids or various suspensions, but also to methods of eliminating the disadvantages caused by air and/or gases existing in and being absorbed into said medium. Especially the invention of the apparatus relates preferably to a construction utilized in connection with a centrifugal pump in order to increase the inlet pressure of the pump.
Prior art knows a large amount of centrifugal pumps that have been and still are used for pumping the fiber suspensions in the wood processing industry. The biggest group is presented by centrifugal pumps having a conventional basic construction with some in-essential changes therein to make them capable of pumping pulp. As an example of this kind of changes, e.g. mounting so-called inducers in front of the actual impeller for facilitating the flow of the pulp to the actual impeller of the pump may be stated. Despite many attempts and minor constructional changes, pumps of the described type are hardly capable of pumping a pulp at a consistency above 6-8%. The reason for this is both the increasing air content of the pulp as the consistency increases, whereby the air or gas bubble accumulated in the center of the impeller prevents the pulp from passing to the impeller, and the poor flow properties of thick pulp in the suction duct of the pump or from the pulp-containing space into the suction duct of the pump.
The second stage, entering the market in the late 1970-""s, was the so-called MC(trademark) pump characterized in that in the inlet opening of the pump there is arranged a rotor most usually extending through the suction duct to some extent into the pulp container, the drop leg or the like, by means of which rotor bonds between fibers of the fiber suspension are being loosened by feeding energy in form of a shear force field into the pulp, whereby the flow of the pulp to the impeller of the pump is facilitated. The objective of these pumps was to make it possible to pump pulps at the consistency of 8-15%. The main problem was considered to be the poor flow properties of pulp at said consistency in the suction duct of the pump, due to which fact the invention was at that time related to methods of getting the pulp to flow in the suction duct of the pump to the impeller. Various embodiments of this kind of pump are described e.g. in U.S. Pat. Nos. 4,410,337, 4,435,193 and 4,637,779. All said solutions are characterized in that they both fluidize the pulp being pumped and remove therefrom gas, most usually air, that disturbs both the pumping and the further treatment of the pulp. The fluidizing is understood to mean breaking the pulp pieces in the fiber suspension into smaller parts to such an extent that the pulp starts behaving as a fluid. The fluidizing is effected by the blades of a rotor located inside the relatively long suction duct of the pump, which blades are located essentially at a radial plane and mainly axially, although some solutions have utilized rotor blades that are twisted to some extent. In all presented pump solutions the separation of gas is effected due to centrifugal force into the hollow center of the rotor in front of the impeller, wherefrom the gas is further removed through openings in the back plate of the impeller in most cases by means of suction created by a vacuum pump. Said suction or vacuum pump, most usually a so-called liquid ring pump, is located either separately from the actual centrifugal pump in connection with a drive of its own or alternatively on the same shaft with the centrifugal pump. As examples of the latter case, e.g. U.S. Pat. Nos. 5,078,573, 5,114,310, 5,116,198, 5,151,010 and 5,152,663 may be mentioned.
About the constructional details of prior art MC(trademark) pumps it may be stated that in all said publications the rotor extends to some extent into the pulp-containing space. Most explicitly this has been described in U.S. Pat. No. 4,637,779, in which the rotor is mentioned to be extending into the tank for about 3 inches, i.e. about 75 mm. This dimension is really true as a maximum range, because the production program mainly includes pumps, the rotor of which does not extend even so deep into the suction chamber. The maximum dimension may be said to be about 0.5*the diameter of the suction duct, which ratio in reality is diminished as the diameter of the suction duct is increased. In practice, the diameter of the smallest MC(trademark) pump is about 150 mm, whereby said ratio is fulfilled. As the diameter of the suction duct further increases, the factual extension of the rotor into the pulp chamber practically does not increase.
Because it was seen from practice, that said extension of the rotor into the chamber was not enough, U.S. Pat. No. 4,971,519 was made to protect a solution, in which the fluidizing rotor was made to extend into the chamber to an extent of at least the length of the diameter of the suction opening of the pump. In an embodiment described in said patent, the end of the fluidizing rotor was provided with blades feeding pulp towards the suction opening of the pump, by which blades a relatively large zone of moving pulp was effected in the vicinity of the suction opening in order to ensure that the pulp would not easily arch in the vicinity of the suction opening.
Now that a lot of practical experience has been gained on said MC(trademark) pumps it has been noticed that the pumps working as such excellently and reaching at their best the consistency ranges up to about 15% can be developed further. The main consumption at the initial stage of developing the MC(trademark) pumps was that the biggest obstacle of pumping a thick pulp is the friction between the wall of the suction duct and the pulp, which friction was attempted to be eliminated by fluidizing the pulp in the suction duct. A second problem was considered to be the discharge of the pulp from the suction chamber or drop leg into the suction duct, because the thick pulp gradually tends to fill the openings surrounded by sharp edges, i.e. including the suction opening. As a result, the fluidizing rotor was decided to be arranged to extend to a certain length into said chamber in order to make the rotor tear off the fibers and fiber flocs possibly attached to the edges of the openings and thus prevent the clogging of the suction opening. However, the old rules self-evident to a designer of centrifugal pumps were maintained, according to which rules the flow of the material being pumped has to be as laminar as possible when entering the pump to eliminate flow losses. References of this kind are still found, e.g. in said U.S. Pat. No. 4,637,779 wherein on column 2, pages 24-30 it is stated that a prior art apparatus generates in front of and around the suction inlet of the pump a xe2x80x9cdoughnut-shapedxe2x80x9d turbulent, i.e. at least partly fluidized, zone which really is located in the vicinity of the edges of the suction inlet of the pump. In said US-publication said phenomena has been considered to disturb the pumping, believing in rules on pump design, and accordingly the tips of the rotor blades extending into the pulp chamber or the like of the MC(trademark) pump have been twisted to give the pulp a force component acting towards the suction inlet. In the publication the utilization of said solution is based on giving the pulp flowing inwards a pressure that facilitates the removing of gas in front of the impeller.
The next confronted problem was the one familiar from pumping pulps of medium consistency by means of MC(trademark) pumps, i.e. even if the pump and its rotor were capable of treating the pulp in the suction duct and further therefrom with adequate efficiency, the problem experienced at consistencies high enough is the getting of the pulp from the pulp chamber or the like into the suction duct. Reasons for this problem are both the arching of the pulp in the pulp space, i.e. the forming of an empty arch-like space in front of the suction inlet of the pump, and the friction between the pulp and the walls of said space, which friction retards the downward flow of the pulp.
Attempts were made to develop the pump according to said U.S. Pat. No. 4,971,519 further to a better direction, because it was noticed that although pulp was no longer arching in front of the pump, the efficiency of the pump was relatively low. As a solution to said problem, U.S. Pat. No. 4,877,368 presented a suction arrangement of a pump wherein there was a screw flight arranged either outside the fluidizing rotor blades of the fluidizing rotor, in the suction duct of the pump or both. The purpose of said flight when attached onto a rotating rotor was to actively feed the pulp towards the impeller of the centrifugal pump, and when attached to the wall of the suction duct to passively guide the pulp flow rotating in the suction duct towards the impeller. Said solution is structurally complicated. It has both essentially axially located fluidizing rotor blades and, in certain embodiments, a flight located on the blades. In other words, producing the rotor as a casting is practically almost impossible.
Experiments of the solution according to said U.S. Pat. No. 4,877,368 have, nevertheless, shown that the development is proceeding to a right direction. But said solution has further disadvantages in addition to a highly complicated and expensive production. As the pitch of the screw arranged on the fluidizing rotor was constant, the pump proved to be very sensible to changes in the volume flow or the rotational speed of the pump. Further, mainly due to said sensibility, it was found out that said pump was applicable to the treatment of pulp at a relatively low consistency only. In practice the upper consistency limit for the pulp was noticed to be about 10 percent, which is too low for almost all applications of the MC(trademark) pumps. Due to said reasons, among others, the pump has never been actively marketed.
The starting point for the next generation high consistency pulp pumps was decided to be the solving of problems described above in such a way that it shall be possible to produce the impeller of the pump by casting and that the pump shall be suitable for pumping volume flows of various amounts at various rotational speeds and that the consistency of the pulp being pumped by said pump shall be essentially higher than 10%. In the experiments performed, a screw-like fluidizer was decided to be used, the pitch of which was changing essentially along the whole length of the screw.
Certainly prior art knows also pumps wherein the pitch of the flight located in front of the impeller of the pump and attached thereto is altering. Mostly these kind of devices are called inducers.
U.S. Pat. No. 4,275,988 deals with a centrifugal pump, in front of the impeller of which there is a screw-like means attached. Said means is formed of a shaft arranged as an extension of the hub of the impeller, to which shaft the flight is attached. The objective of said screw-like means is to increase the suction capability of the pump either with high-speed pumps or in situations where the suction head of the pump is low. As examples of applications for use, e.g. chemical and petrochemical industries are mentioned. The main problem is considered to be the high cavitation susceptibility of known pumps as well as great pressure fluctuations in the suction and pressure ducts. The starting point in said publication is that according to the principle of geometrical equality, the diameter and pitch of said screw-like feeding apparatus have to change in the same ratio. In other words, as the diameter of the screw doubles, the pitch must also double. The publication presents a number of various embodiments to fulfill said initial requirement. The solutions presented in the publication are also characterized in that the rotor is in no way dimensioned in correspondence to the suction duct, but only the diameter and the pitch of the rotor are mutually adjusted as described before. The result is that with a small rotor diameter, the distance between the rotor and the suction duct wall is relatively long. That questions the feeding effect of the rotor, especially with stiff materials, as the rotor only opens a cavity in the stiff material without forcing it to flow into the suction duct and therefrom to the pump.
CH patent publication 606 804 also deals with a centrifugal pump with a screw-like feeding member arranged as an extension of the impeller. In this case, also, the flights of the member have been attached onto the shaft functioning as an extension of the hub of the impeller. The different embodiments of the publication present several various feeding member constructions. These are all characterized in that they are completely located inside the suction duct of the pump and in that they leave a relatively long free zone between themselves and the impeller, to which zone neither the rotor nor the impeller extends. Further, concluding from the solutions of the publication, the distance between the rotor and the suction duct of the pump is not essential for said devices, because e.g. FIGS. 5 and 7 of the publication illustrate a rotor with a remarkably small diameter. In addition to that, the solutions of the publication present that the rotor part may be provided with screws with a pitch of two different orders of magnitude (FIGS. 6 and 7). The publication is concentrated especially on methods of decreasing the noise caused by these so-called inducers, particularly at partial pump loading.
To put it differently, prior art inducer solutions utilizing a continuous flight for feeding a medium to a centrifugal pump, always comprise a shaft located on the axis of the suction duct of the pump which shaft naturally closes the center of the suction duct. This kind of solution is not the best possible one for pumping a medium containing gas or material easily changing into a gas-like condition (vaporizing) (e.g. hot water), because the existing shaft prevents effective separation of gas or vapor into the center of the flow. Thus it is clear that said prior art pumps have never been presented for pumping a liquid containing gas-like material, but for pumping liquid only. This becomes obvious, among other things, from the fact that in no prior art pump with this kind of closed inducer with a closed center, the impeller is provided with openings for gas-removal.
The objective of the present apparatus and method according to the invention is to solve at least part of said problems disturbing prior art pumps. As some characterizing features of the invention e.g. the following may be mentioned:
in a preferred embodiment a fluidizing rotor with an open center,
a separation arrangement for gas and/or vapor in connection with the rotor and/or the impeller,
fluidizing rotor blades, the pitch of which changes essentially evenly essentially on the whole length of the rotor, and
a clear gap between the rotor and the suction duct.
The method and apparatus according to the invention are well suitable for pumping various liquids. As examples of these mediums at least the following are worth mentioning: gas-containing pulps (e.g. fiber suspensions of the wood processing industry), especially hot pulps, process filtrates, chips, other easily vaporizing liquids of the cellulose, sugar and food industry and different hot liquids. In addition to that, the method and apparatus according to the invention have made it possible to pump all said mediums at a higher temperature than before.
The method of pumping a gas-containing and/or viscous material according to the invention by means of an apparatus mainly comprising a casing, suction and discharge ducts therein, an impeller including at least one or more pumping vanes and a rotor arranged in front of the impeller, which rotor further comprises one or more blades, in which method said material is made to flow into the pumping apparatus through said suction duct, the material is discharged into the discharge duct, is characterized in that at the beginning part of the suction duct, viewing from the impeller at its further end, the pressure of the pulp is raised in order to feed the pulp into the apparatus.
The apparatus according to the invention for pumping a gas-containing and/or viscous material, which apparatus mainly comprises a casing, suction and discharge ducts therein, an impeller comprising at least one or more pumping vanes, and a rotor arranged in front of the impeller, which rotor further comprises one or more blades, is characterized in that the blades of said rotor have been twisted so that their pitch changes along an essential part of the length of the rotor.
The rotor according to the invention for use in connection with an apparatus mainly comprising a casing, suction and discharge ducts therein and an impeller having at least one or more pumping vanes for pumping a gas-containing and/or viscous material, which rotor comprises one or more blades, is characterized in that the blades of said rotor have been twisted so that their pitch changes along an essential part of the length of the rotor.
Other characterizing features of the method and apparatus according to the invention are disclosed in the appended claims.